Bone anchoring device

ABSTRACT

A bone anchoring device includes a shank to be anchored in a bone or vertebra, a head, and a receiving part receiving the head for connecting the shank to a rod. The shank and the head are separate parts. The head has a bore with a cylindrical inner surface, a ring-shaped groove provided on the inner surface, and at least one slit extending from the inner surface to the outer surface of the head. The shank has a first portion for anchoring in the bone or vertebra and a second portion with a cylindrical outer surface adjoining a free end. A projection is provided on the cylindrical outer surface which engages with the ring-shaped groove of the head, when the cylindrical second portion of the shank is inserted in the bore of the head.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/860,958, filed Nov. 22, 2006, and claimspriority from European Patent Application EP06024232.8, filed Nov. 22,2006, the disclosures of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a bone anchoring device, and moreparticularly to a polyaxial bone anchoring device having a shank and ahead formed as separate parts.

U.S. Pat. No. 6,835,196 B2 describes an anchoring element for connectinga screw to a rod. The shank of the screw and the head are formed asseparate parts. The head is shaped as a segment of a sphere which ispivotably held in a receiving portion. In one example the end portion ofthe shank has a cylindrical shape and the head is provided with a slitand has a cylindrical bore into which the cylindrical end portion of theshank is inserted.

US 2005/0049588 A1 discloses a polyaxial bone screw for connection to aspinal fixation rod. At one end the shank of the bone screw has afrusto-conical capture structure diverging in diameter in a directionaway from the threaded end of the shank. A retainer ring has a centralbore with a conical shape which is compatible with the conical surfaceof the shank for receiving the shank. The angular orientation of theshank relative to a receiving part is fixed by the spinal fixation rodacting on an end portion of the shank.

US 2006/0036252 A1 discloses a polyaxial screw which has a threadedshaft for securing the screw to bone. One end of the shaft has a reversetapered head tapering toward and away from the shaft from an enlargedmedian. A spherical shaped swivel, having an opening smaller than themedian, is snapped over the head to rotatably engage the shaft.

Based on the foregoing, there is a need for a bone anchoring device witha shank and a head formed as separate parts which allows convenient andreliable handling during assembly of the shank and the head, providessecure locking of the shank in the head after assembly, and provides amodular system having variability.

SUMMARY

In accordance with aspects of the disclosure, since the shank and thehead are formed as separate parts, in use of the bone anchoring device,an appropriate shank can be selected depending on the application andcan be connected to the head for forming the bone anchoring device. Inparticular, shanks of different lengths can be provided in combinationwith the head such that the costs for stock keeping can be reduced.

The cylindrical shape of the end portion of the shank cooperating withthe cylindrical bore in the head allows convenient and reliableinsertion of the shank into the head. Easy insertion is further improvedby at least one slit provided in the head which imparts a certainflexibility to the head. Engagement of a projection with a correspondinggroove during assembly leads to the result that the shank is securelyretained in the head in an assembled condition.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will arise from thedescription of embodiments with reference to the enclosed figures, ofwhich:

FIG. 1 is a perspective view of a bone anchoring device according to afirst embodiment in an assembled condition;

FIG. 2 is an exploded view of the bone anchoring device of FIG. 1;

FIG. 3 is a sectional view of the bone anchoring device of FIG. 1;

FIG. 4 a to 4 c are illustrations of the shank (FIG. 4 a), the head(FIG. 4 b), and a snap ring (FIG. 4 c) of the first embodiment;

FIG. 5 is a perspective view for illustrating the assembly of the boneanchoring device according to the first embodiment;

FIG. 6 is a perspective view of the shank and the head of a secondembodiment;

FIG. 7 is an exploded view of the embodiment of FIG. 6;

FIG. 8 is a sectional view of the upper part of the embodiment shown inFIG. 6 in an assembled state;

FIG. 9 is an exploded sectional view of the shank and the head of thesecond embodiment;

FIG. 10 is an exploded view of a third embodiment; and

FIG. 11 is a sectional view of the upper part of the third embodiment inan assembled state.

DETAILED DESCRIPTION First Embodiment

A first embodiment will be described with reference to FIGS. 1 to 4. Ascan be seen in the exploded view of FIG. 2, the bone anchoring element 1of the first embodiment includes a shank 2, a head 3 provided separatelyfrom the shank 2, a receiving part 4 and a pressure element 5. Further,in the first embodiment a ring 7 is provided which will be described indetail below.

As can be seen in FIG. 4 a, the shank 2 comprises a tip 20, a firstportion 21 which is provided with a bone thread 22, and a second portion23 adjacent to the free end 27 opposite to the tip 20 which has acylindrical surface. The second portion 23 has a substantially smoothsurface. Further, a circumferential groove 24 is provided in the outersurface of the second portion 23. The free end 27 of the second portion23 has a beveled shape such that the diameter tapers towards the freeend 27.

As can be seen in FIG. 3, the shank 2 is further provided with a coaxialbore 25. The bore 25 is open at the free end 27 and extends to apredetermined distance from the tip 20. Lateral through-holes 26 can beprovided in the first portion 21 which extend from the bore 25 to theouter surface of the first portion. The through-holes 26 can be arrangedin the base of the bone thread 22 between the thread crests. The bore 25and the through-holes 26 allow, for example, introduction of bone cementto enhance the fixation of the shank 2 in the bone. The free end 27 ofthe shank further includes a recess for engagement with a screwing-intool.

The head 3 has a substantially sleeve-like shape with a first end 32 anda second end 33. The outer surface 30 is shaped as a segment of asphere. A cylindrical bore 31 is provided which extends through the headfrom the first end 32 to the second end 33. A circumferential groove 34is provided approximately at the center of the cylindrical bore 31. Ascan be seen in FIGS. 3 and 4 b, the groove 34 has a substantiallyrectangular cross section.

Further, a longitudinal slit 35 is provided in the head 3 which extendsfrom the first end 32 to the second end 33. Although only one slit 35which extends from the first end 32 to the second end 33 is shown in theembodiment, the slit does not have to extend continuously from the firstend 32 to the second end 33. One slit extending from the first end 32 orthe second end 33 to a predetermined distance from the respective otherend may be provided. As an alternative, for example, several slits whichalternately extend from the first end 32 to a predetermined distancefrom the second end 33 and from the second end 33 to a predetermineddistance from the first end 32 may be provided. The slit in the head 3serves to provide elasticity to the head 3 in order to allow insertionof a slightly oversized shank, as will be described below, and toprovide secure clamping after final locking with a securing element.

The ring 7 is provided as a snap ring which has a substantially annularshape with a clearance 70, as can be seen in FIG. 4 c. The ring 7 has asubstantially rectangular cross section and is shaped such that it fitsin the groove 24 on the second portion 23 of the shank 2 and in thegroove 34 in the cylindrical bore 31 of the head 3. Due to the clearance70 the ring 7 has certain flexibility, i.e. its diameter can be enlargedand decreased by a certain amount.

As can be seen in FIGS. 2 and 3, the receiving part 4 has asubstantially cylindrical shape with a first end 40 and a second end 41and a longitudinal axis Z. A coaxial bore 42 extends through thereceiving part 4 from the first end 40 to the second end 41. The shapeof the longitudinal bore 42 tapers towards the first end 40 such that aseat 43 for accommodating the outer surface 30 of the head 3 is formed.The seat can have, for example, a spherical shape or a conical shape oranother shape with a slightly reduced diameter of the bore, which thusprovides an efficient end stop preventing the head from easily fallingout off the bore. Further, a substantially U-shaped recess 44 is formedin the receiving part 4 adjacent to the second end 41 such that two freelegs 45 and 46 are formed which form a channel extending substantiallyperpendicular to the bore 42 for receiving a rod 60. On the innersurface of the free legs 45, 46 adjacent to the second end 41 an innerthread 47 is provided for screwing-in a securing element 61 for securingthe rod 60 in the U-shaped recess. In the embodiment the inner thread 47is designed as a flat thread having the thread flanks arrangedperpendicular to the longitudinal axis. However, the thread can have anyother suited thread shape.

Further, as shown in FIG. 2 the pressure element 5 has a substantiallycylindrical shape and is dimensioned such that it can be inserted in thecoaxial bore 42 of the receiving part 4 and is slidably guided therein.On its upper side the pressure element 5 has a recess 52 shaped as asegment of a hollow cylinder the diameter of which substantiallycorresponds to the diameter of the bottom of the U-shaped recess 44 ofthe receiving part 4. On the lower side 51 of the pressure element 5 arecess (not shown) shaped as a segment of a hollow sphere is providedthe diameter of which substantially corresponds to the outer diameter ofthe head 3. A bore 53 is provided in the pressure element 5 coaxially toits cylinder axis in order to allow guiding through of a screwing-intool. In use, the pressure element 5 is placed in the longitudinal bore42 such that the recess 52 shaped as a segment of a hollow cylinder isaligned with the U-shaped recess 44, as can be seen in FIG. 1.

The operation of the bone anchoring device 1 of the first embodimentwill now be described with reference to FIG. 5. In operation, first theshank 2 can be screwed into a bone or vertebra by engagement with ascrewing-in tool. The receiving part 4, the head 3, the ring 7 and thepressure element 5 can be provided in a preassembled state. In thepre-assembled state the ring 7 is inserted in the cylindrical bore 31 ofthe head 3 such that it rests in the ring-shaped groove 34. Further, thehead 3 is placed in the longitudinal bore 42 of the receiving part 4such that it rests on the seat 43 and remains pivotable with respect tothe receiving part 4. The pressure element 5 is placed in thelongitudinal bore 42 of the receiving part 4 such that the recess on thelower side 51 faces the outer surface 30 of the head 3 and such that therecess 52 shaped as a segment of a hollow cylinder is aligned with theU-shaped recess 44. In this assembled state, due to the flexibilityprovided by the clearance 70, the ring 7 can still be expanded such thatits diameter is enlarged. Further, due to the slit 35, the head 3 canstill be expanded such that its diameter is enlarged.

The assembly of receiving part 4, head 3, ring 7 and pressure element 5can then be connected to the shank 2 by guiding the second portion 23 ofthe shank 2 into the cylindrical bore 31 of the head 3. The beveledshape of the upper free end 27 of the shank 2 facilitates insertion.During insertion the head 3 and the ring 7 are slightly expanded by thesecond portion 23 of the shank until the ring 7 snaps into the groove 24of the second portion 23 of the shank. In this condition the ring 7rests in the groove 24 of the end portion of the shank 2 and in thegroove 34 of the head 3. As a result, the shank 2 is prevented fromretracting again from the head 3 by form-locking engagement of the ring7 in the groove 34 in the head 3.

The cylindrical shape of the second portion 23 of the shank 2 allowseasy and convenient insertion of the shank 2 into the head 3, theprojection formed by the ring 7 arranged in the groove 24 on the shank 2allows secure fixation of the shank 2 to the head 3 by form-locking.

After insertion of the shank 2 in the head 3, the angular orientation ofthe shank 2 relative to the cylinder axis Z of the receiving part 4 canstill be varied. In operation then a rod can be placed in the U-shapedrecess 44 such that it rests in the recess 52 shaped as a segment of ahollow cylinder of the pressure element 5. A securing element can bescrewed-in between the legs 45, 46 by cooperating with the inner thread47. The securing element exerts pressure on the rod and the rod in turnexerts pressure on the pressure element 5. As a result, the pressureelement 5 presses on the head 3 and the angular orientation of the head3 relative to the receiving part 4 is thereby fixed. Further, since thehead 3 is pressed against the seat 43 which is formed in a taperedshape, the head 3 is slightly compressed and the connection of the shank2 to the head 3 is further strengthened.

The slit in the head 3 provides certain elasticity to the head 3, thus,a shank 2 having a slightly over-dimensioned second portion 23 can beconnected to the head 3. Further, this elasticity provides secureclamping of the shank 2 in the head 3, when the securing element isfastened.

As described above, the shank 2 is first screwed into the bone orvertebra before connecting it to the head 3. Alternatively, the shank 2can first be connected to the head 3 and the receiving part, such thatthe anchoring element is in a pre-assembled state, and thereafter theshank 2 can be screwed into the bone or vertebra by guiding ascrewing-in tool through the longitudinal bore 42 of the receiving part4 and through the bore 53 of the pressure element 5 into the structurein the longitudinal bore 25 of the shank 2. Thus, variability isensured.

Second Embodiment

A second embodiment will be described with reference to FIGS. 6 to 9.The second embodiment differs from the first embodiment in the structureof the shank and the head. The receiving part 4 and the pressure element5 are identical to those of the first embodiment and will not bedescribed again. The receiving part 4 and the pressure element 5 areomitted in FIGS. 6 to 9.

The shank 102 according to the second embodiment comprises a firstportion 121 which is provided with a bone thread 122 and a secondportion 123 having a cylindrical outer surface with a first diameterwherein the surface is substantially smooth. As can be seen in FIGS. 8and 9, the shank 102 does not have a longitudinal bore as the shank inthe first embodiment. A recess 126 for engagement with a screwing-intool is provided at the free end 127. As in the first embodiment thefree end 127 of the shank 102 has a beveled shape to facilitateinsertion of the shank 102 into the head 103.

A circumferentially extending projection 107 is formed on the secondportion 123. The projection 107 is formed integrally with the secondportion 123

Similar to the first embodiment, the head 103 has a substantiallysleeve-like shape. The outer surface 130 is shaped as a segment of asphere and cylindrical bore 131 is provided. A circumferential groove134 is provided approximately at the center of the cylindrical bore 131.

Further, a plurality of longitudinal slits 135 are provided in the head103 which alternately extend from the first end 132 to a predetermineddistance from the second end 133 and from the second end 133 to apredetermined distance from the first end 132. This structure of theslits 135 provides a uniform flexibility to the head 103 in the regionof the first end 132 and in the region of the second end 133.

Operation of the second embodiment is similar to the first embodiment.When the free end 127 of the shank 102 is inserted into the cylindricalbore 131 of the head 103, the diameter of the head 103 becomes enlargedby introduction of the circular projection 107. When the projection 107reaches the ring-shaped groove 134 in the cylindrical bore 131, the head103 elastically contracts and the projection 107 rests in the groove 134such that the shank 102 is prevented from retracting from thecylindrical bore 131. After assembly of the complete bone anchoringdevice the pressure transmitted via the pressure element 5 furtherenhances the fixation of the shank 102 in the head 103 similar to thefirst embodiment.

According to the first embodiment, a projection projecting from thecylindrical outer surface of the second portion 23 of the shank 2 isformed by the ring 7 which is placed in a groove 24 in the shank 2.Similarly in the second embodiment a projection projecting from thecylindrical outer surface of the second portion 123 of the shank 102 isformed by the circular projection 107 which is integrally formed withthe second portion 123. In both cases, easy and convenient insertion ofthe shank 2, 102 into the cylindrical bore 31, 131 of the head 3, 103 isallowed due to the cylindrical shape of the outer surface of the secondportion 23, 123. Further, in both cases the shank 2, 102 is securedagainst unintended retraction by means of the projection 7, 107projecting from the outer surface of the second portion 23, 123 engagingwith the groove 34, 134 provided in the head 3, 103. The slits 35, 135provide flexibility to the respective head 3, 103 which furtherfacilitates insertion.

Third Embodiment

A third embodiment is shown in FIGS. 10 and 11. The third embodimentdiffers from the first and second embodiments in the structure of thehead and the pressure element. The further structure is similar to thatof the first and second embodiments and will not be described again.

The bone anchoring devices of the first and second embodiments providesa so-called polyaxial connection of the shank 2, 102 and the receivingpart 4. In contrast, the bone anchoring device according to the thirdembodiment provides a monoaxial connection between a shank 202 and thereceiving part 4. The shank 202 is constructed similar to the shank 2 inthe first embodiment with a first portion 221, a cylindrical secondportion 223 adjacent to a free end 227, and with a groove 224, butwithout a longitudinal bore and without through-holes.

As can be seen in FIG. 10, in the third embodiment the head and thepressure element are not constructed as separate parts but are formed inone piece as a head 203. The head 203 has a substantially cylindricalshape with a cylindrical outer surface 230 dimensioned such that it canbe inserted in the longitudinal bore 42 of the receiving part 4. At afirst end 203 a the outer diameter of the head 203 tapers in a sphericalshape. The opposite second end 203 b is formed similar to the upper side50 of the pressure element 5 in the first embodiment, i.e. acylindrically-shaped recess 252 is formed at the second end 203 b. Aplurality of slits 235 extend from the first end 203 a to apredetermined distance from the second end 203 b. The slits 235 serve toprovide flexibility to the head 203. Again, only one slit 235 may beprovided alternatively.

The head 203 has a cylindrical bore 231 and a ring-shaped groove 234similar to the head 3 in the first embodiment.

Assembly of the third embodiment is similar to the first embodiment. Inoperation, the head 203 is inserted in the longitudinal bore 42 of thereceiving part 4. The snap ring 7 is placed in the groove 224 on thesecond portion 223. Then, the cylindrical second portion 223 of theshank 202 is inserted into the cylindrical bore 231 of the head 203 andthe head 203 slightly widens due to the flexibility provided by theslits 235. The shank 202 is inserted until the snap ring 7 faces thering-shaped groove 234 provided in the head 203 and then, due to theflexibility of the head 203, the snap ring 7 engages with the groove234. As a result, the shank 202 is prevented from retracting again fromthe head 203.

After insertion of the rod 60 the securing element 61 is inserted forlocking the rod 60 in the receiving part 4. During tightening of thesecuring element 61 pressure acts on the second end 203 b of the head203. The head 203 is pressed towards the first end 40 of the receivingpart 4. Due to the tapering shape of the longitudinal bore 42 and to theslits 235 provided in the head 203, the head 203 is compressed and theshank 202 becomes securely locked in the head 203.

In an alternative way of operation the snap ring can first be placed inthe ring-shaped groove 234 of the head 203 and thereafter the shank 202can be inserted.

Further Modifications

In a further modification of the polyaxial bone anchoring device theshank is formed similarly to the shank 2 in the first embodiment with agroove 24 provided in the cylindrical outer surface of the secondsection 23. In contrast to the first embodiment the ring 7 is omittedand, instead of the ring-shaped groove 34 in the cylindrical bore 31 ofthe head 3, a ring-shaped projection with a reduced diameter as comparedto the cylindrical bore 31 is integrally formed in the cylindrical bore31. When the shank 2 is inserted in the cylindrical bore 31, thisring-shaped projection engages with the groove 24 of the shank 2.

As a result, a similar connection as that described with respect to thefirst and the second embodiments can be achieved. This structure canalso be applied to the monoaxial connection shown in the thirdembodiment.

The ring 7 in the first and third embodiment can be considered to form aprojection provided on the cylindrical inner surface of the bore 31,231. The projection formed by the ring 7 engages with the groove 24, 224provided on the shank 2, 202.

The structure of the projection 107 of the second embodiment cooperatingwith a groove provided in the head can be applied to the monoaxialconnection of the third embodiment.

Although embodiments have been described in which locking of the rod andthe angular orientation of the shank in the polyaxial bone anchoringdevice is performed by one single securing element, it is also possibleto provide two securing elements for independently fixing the rod andthe head. Alternatively, the securing element can be formed by an outernut which is screwed on an outer thread provided on the free legs of thereceiving part.

In the embodiments above the pressure element has a groove in the uppersurface. However, it is also possible to provide a pressure element witha flat upper surface.

Further modifications are possible. For example the features of the headaccording to the second embodiment can be combined with the shankaccording to the first embodiment and vice versa.

Different types of shanks with or without coaxial bores and withdifferent types of bone threads can be provided such that a modularsystem is formed. In such a system, also hooks may be provided. Further,different types of heads may be provided which can be combined with theshanks depending on the respective application. In this way, a modularsystem can be provided which offers variability in application.

While a particular form of the disclosure has been illustrated anddescribed, it will be apparent that various modifications can be madewithout departing from the spirit and scope of the disclosure.Accordingly, it is not intended that the disclosure be limited, exceptas by the appended claims.

What is claimed is:
 1. A polyaxial bone anchoring device comprising: ashank to be anchored in a bone or vertebra, a head, a ring, and areceiving part receiving the head for connecting the shank to a rod, theshank, the head, and the ring being separate parts, the head having abore with a cylindrical inner surface defining a smallest diameter ofthe bore, the cylindrical inner surface extending uniformly from a firstend of the head to a second end of the head and interrupted by aring-shaped groove provided on the inner surface, and at least one slitextending from the inner surface to an outer surface of the headproviding elasticity to the head; the shank having a first portion foranchoring in the bone or vertebra and a second portion with acylindrical outer surface, wherein a ring-shaped groove is provided onthe cylindrical outer surface; wherein the ring is configured to bearranged in the groove of the head or the shank; wherein in a firstinserted position, the ring is positioned in the grooves of the head andthe shank such that a surface of the ring engages a correspondingsurface on the head or the shank, the engaging surfaces configured toform a positive-locking engagement that prevents sliding between saidengaging surfaces and removal of the shank from the bore of the headwhen a force is applied to the shank in a direction opposite a directionof insertion; wherein the receiving part has a longitudinal bore passingthrough it from a first end to a second end, the longitudinal borehaving a reduced diameter adjacent to the first end of the receivingpart forming a seat for accommodating the head, the receiving partfurther having a U-shaped recess adjacent to the second end of thereceiving part, the U-shaped recess intersecting the longitudinal boresuch that two free legs are formed constituting a channel for receivingthe rod; wherein the shank is insertable into the head while the head isarranged in the receiving part; and wherein a pressure element isarranged in the longitudinal bore and configured to fix the angularorientation of the shank relative to the receiving part, the pressureelement acting onto the head such that it compresses the head whichfirmly clamps the head to the shank.
 2. The polyaxial bone anchoringdevice according to claim 1, wherein the outer surface of the head issubstantially shaped as a segment of a sphere.
 3. The polyaxial boneanchoring device according to claim 1, wherein the at least one slitextends from a first end of the head to a second end of the head.
 4. Thepolyaxial bone anchoring device according to claim 1, wherein severalslits are provided in the head which alternately extend from a first endof the head to a predetermined distance from a second end of the headand from the second end of the head to a predetermined distance from thefirst end of the head, respectively.
 5. The polyaxial bone anchoringdevice according to claim 1, wherein the U-shaped recess extendssubstantially perpendicular to the longitudinal bore.
 6. The polyaxialbone anchoring device according to claim 1, wherein when the ring isprovided on the inner surface of the head, the ring is configured toengage with the ring-shaped groove of the shank when the second portionof the shank is inserted in the bore of the head.
 7. The polyaxial boneanchoring device according to claim 1, wherein the ring-shaped grooveprovided on the cylindrical inner surface is spaced apart fromrespective portions of the cylindrical inner surface adjacent to thefirst end and the second end of the head.
 8. The polyaxial boneanchoring device according to claim 1, wherein the cylindrical outersurface of the second portion of the shank extends from a first end to asecond end, and the ring-shaped groove provided on the cylindrical outersurface is between the first end and the second end of the cylindricalouter surface and spaced apart from the first end and the second end ofthe cylindrical outer surface.
 9. The polyaxial bone anchoring deviceaccording to claim 1, wherein when the ring is arranged in the groove ofthe head, the ring is configured to expand in the head upon engagementwith the shank.
 10. The polyaxial bone anchoring device according toclaim 1, wherein the ring comprises a snap ring having a slit.
 11. Thepolyaxial bone anchoring device according to claim 1, wherein saidengaging surface of the ring and said corresponding engaging surface onthe head or the shank are matching surfaces.
 12. The polyaxial boneanchoring device according to claim 1, wherein said engaging surface ofthe ring extends in a plane perpendicular to said direction ofinsertion.
 13. A method of attaching a polyaxial bone anchoring deviceto a bone or vertebra, the polyaxial bone anchoring device comprising ashank to be anchored in a bone or vertebra, a head, a ring, and areceiving part receiving the head for connecting the shank to a rod, theshank, the head, and the ring being separate parts, the head having abore with a cylindrical inner surface defining a smallest diameter ofthe bore, the cylindrical inner surface extending uniformly from a firstend of the head to a second end of the head and interrupted by aring-shaped groove provided on the inner surface, and at least one slitextending from the inner surface to an outer surface of the headproviding elasticity to the head, the shank having a first portion foranchoring in the bone or vertebra and a second portion with acylindrical outer surface, wherein a ring-shaped groove is provided onthe cylindrical outer surface, wherein the ring is configured to bearranged in the groove of the head or the shank, wherein in a firstinserted position, the ring is positioned in the grooves of the head andthe shank such that a surface of the ring engages a correspondingsurface on the head or the shank, the engaging surfaces configured toform a positive-locking engagement that prevents sliding between saidengaging surfaces and removal of the shank from the bore of the headwhen a force is applied to the shank in a direction opposite a directionof insertion, wherein the receiving part has a longitudinal bore passingthrough it from a first end to a second end, the longitudinal borehaving a reduced diameter adjacent to the first end of the receivingpart forming a seat for accommodating the head, the receiving partfurther having a U-shaped recess adjacent to the second end of thereceiving part, the U-shaped recess intersecting the longitudinal boresuch that two free legs are formed constituting a channel for receivingthe rod, wherein the shank is insertable into the head while the head isarranged in the receiving part, and wherein a pressure element isarranged in the longitudinal bore and configured to fix the angularorientation of the shank relative to the receiving part, the pressureelement acting onto the head such that it compresses the head whichfirmly clamps the head to the shank, the method comprising: attachingthe shank to the bone or vertebra; inserting the head in the bore of thereceiving part to seat the head at the second end of the receiving part;connecting the head to the shank by inserting the second portion of theshank in the bore of the head, wherein when the ring is provided on theinner surface of the head, the connecting of the head and the shankcomprises the ring provided on the inner surface of the head engagingthe ring-shaped groove on the outer surface of the shank in the firstinserted position; and inserting the pressure element in the receivingpart to fix the angular orientation of the shank relative to thereceiving part, the pressure element acting onto the head such that itcompresses the head which firmly clamps the head to the shank.
 14. Apolyaxial bone anchoring system comprising: a plurality of shanks, anyone of the shanks to be anchored in a bone or vertebra, at least onehead, at least one ring, and a receiving part receiving the head forconnecting any one of the shanks to a rod, the shanks, the head, and thering being separate parts, the head having a bore with a cylindricalinner surface defining a smallest diameter of the bore, the cylindricalinner surface extending uniformly from a first end of the head to asecond end of the head and interrupted by a ring-shaped groove providedon the inner surface, and at least one slit extending from the innersurface to an outer surface of the head providing elasticity to thehead; each shank having a first portion for anchoring in the bone orvertebra and a second portion with a cylindrical outer surface, whereina ring-shaped groove is provided on the cylindrical outer surface;wherein the ring is configured to be arranged in the groove of the heador one of the shanks; wherein in a first inserted position, the ring ispositioned in the grooves of the head and any one of the shanks suchthat a surface of the ring engages a corresponding surface on the heador the shank, the engaging surfaces configured to form apositive-locking engagement that prevents sliding between said engagingsurfaces and removal of the shank from the bore of the head when a forceis applied to the shank in a direction opposite a direction ofinsertion; wherein the receiving part has a longitudinal bore passingthrough it from a first end to a second end, the longitudinal borehaving a reduced diameter adjacent to the first end of the receivingpart forming a seat for accommodating the head, the receiving partfurther having a U-shaped recess adjacent to the second end of thereceiving part, the U-shaped recess intersecting the longitudinal boresuch that two free legs are formed constituting a channel for receivingthe rod; wherein any one of the shanks is insertable into the head whilethe head is arranged in the receiving part; and wherein a pressureelement is arranged in the longitudinal bore and configured to fix theangular orientation of any one of the shanks relative to the receivingpart, the pressure element acting onto the head such that it compressesthe head which firmly clamps the head to any one of the shanks.
 15. Thepolyaxial bone anchoring system according to claim 14, wherein theshanks have different bone threads.
 16. The polyaxial bone anchoringsystem according to claim 14, wherein at least one of the shankscomprises a coaxial bore and lateral through-holes extending from thebore to the outer surface of the shank.
 17. The polyaxial bone anchoringsystem according to claim 14, further comprising a plurality of heads,each head having a bore with a cylindrical inner surface defining asmallest diameter of the bore, the cylindrical inner surface extendinguniformly from a first end of the head to a second end of the head andinterrupted by a ring-shaped groove provided on the inner surface, andat least one slit extending from the inner surface to the outer surfaceof the head providing elasticity to the head, wherein each head isconnectable with any one of the shanks.
 18. The polyaxial bone anchoringsystem according to claim 17, wherein the at least one slit of at leastone of the heads extends from a first end of the head to a second end ofthe head.
 19. The polyaxial bone anchoring system according to claim 17,wherein several slits are provided in at least one of the heads whichalternately extend from a first end of the head to a predetermineddistance from a second end of the head and from the second end of thehead to a predetermined distance from the first end of the head,respectively.
 20. The polyaxial bone anchoring system according to claim14, wherein when the ring is provided on the inner surface of the head,the ring is configured to engage with the ring-shaped groove of any oneof the shanks when the second portion of the shank is inserted in thebore of the head.
 21. A bone anchoring device comprising: a shank to beanchored in a bone or vertebra, a head, a ring, and a receiving partreceiving the head for connecting the shank to a rod; the shank, thehead, and the ring being separate parts; the head having a bore with acylindrical inner surface defining a smallest diameter of the bore, thecylindrical inner surface extending uniformly from a first end of thehead to a second end of the head and interrupted by a ring-shaped grooveprovided on the inner surface, and at least one slit extending from theinner surface to an outer surface of the head; and the shank having afirst portion for anchoring in the bone or vertebra and a second portionwith a cylindrical outer surface, the cylindrical outer surface having aring-shaped groove; wherein the ring is configured to be arranged in thering-shaped groove of the head; wherein a free end of the second portionof the shank comprises an inwardly tapered edge portion; wherein whenthe ring is arranged in the groove of the head, the ring is configuredto engage the free end of the shank to expand the ring by the inwardlytapered edge portion, and then to engage with the ring-shaped groove ofthe shank in a first inserted position, when the second portion of theshank is inserted in the bore of the head, the shank being insertableinto the head while the head is arranged in the receiving part; whereinin the first inserted position, the ring is positioned in the grooves ofthe head and the shank such that a surface of the ring engages acorresponding surface on the head or the shank, the engaging surfacesconfigured to form a positive-locking engagement that prevents slidingbetween said engaging surfaces and removal of the shank from the bore ofthe head when a force is applied to the shank in a direction opposite adirection of insertion; and wherein the receiving part has alongitudinal bore passing through it from a first end to a second end,and a U-shaped recess adjacent to the second end and intersecting thelongitudinal bore such that two free legs are formed constituting achannel for receiving the rod.
 22. The bone anchoring device accordingto claim 21, wherein the ring has a rectangular cross-sectional profile.23. The bone anchoring device according to claim 21, wherein the outersurface of the head is substantially shaped as a segment of a sphere.24. The bone anchoring device according to claim 21, wherein the atleast one slit extends from a first end of the head to a second end ofthe head.
 25. The bone anchoring device according to claim 21, whereinseveral slits are provided in the head which alternately extend from afirst end of the head to a predetermined distance from a second end ofthe head and from the second end of the head to a predetermined distancefrom the first end of the head, respectively.
 26. The bone anchoringdevice according to claim 21, wherein the longitudinal bore has areduced diameter adjacent to the first end of the receiving part forminga seat for accommodating the head.
 27. The bone anchoring deviceaccording to claim 26, wherein a pressure element is arranged in thelongitudinal bore for fixing the angular orientation of the shankrelative to the receiving part.
 28. The bone anchoring device accordingto claim 26, wherein the U-shaped recess extends substantiallyperpendicular to the longitudinal bore.
 29. The bone anchoring deviceaccording to claim 21, wherein the ring-shaped groove provided on thecylindrical inner surface is spaced apart from respective portions ofthe cylindrical inner surface adjacent to the first end and the secondend of the head.
 30. The bone anchoring device according to claim 21,wherein the cylindrical outer surface of the second portion of the shankextends from a first end to a second end, and the ring-shaped grooveprovided on the cylindrical outer surface is between the first end andthe second end of the cylindrical outer surface and spaced apart fromthe first end and the second end of the cylindrical outer surface. 31.The bone anchoring device according to claim 21, wherein when the ringis arranged in the groove of the head, the ring is configured to expandin the head upon engagement with the shank.
 32. The bone anchoringdevice according to claim 21, wherein the ring comprises a snap ringhaving a slit.
 33. The bone anchoring device according to claim 21,wherein said engaging surface of the ring and said correspondingengaging surface on the head or the shank are matching surfaces.
 34. Thebone anchoring device according to claim 21, wherein said engagingsurface of the ring extends in a plane perpendicular to said directionof insertion.
 35. A method of attaching a bone anchoring device to abone or vertebra, the bone anchoring device comprising a shank to beanchored in a bone or vertebra, a head, a ring, and a receiving partreceiving the head for connecting the shank to a rod, the shank, thehead, and the ring being separate parts, the head having a bore with acylindrical inner surface defining a smallest diameter of the bore, thecylindrical inner surface extending uniformly from a first end of thehead to a second end of the head and interrupted by a ring-shaped grooveprovided on the inner surface, and at least one slit extending from theinner surface to an outer surface of the head, and the shank having afirst portion for anchoring in the bone or vertebra and a second portionwith a cylindrical outer surface, the cylindrical outer surface having aring-shaped groove, wherein the ring is configured to be arranged in thering-shaped groove of the head, wherein a free end of the second portionof the shank comprises an inwardly tapered edge portion, wherein whenthe ring is arranged in the groove of the head, the ring is configuredto engage the free end of the shank to expand the ring by the inwardlytapered edge portion, and then to engage with the ring-shaped groove ofthe shank in a first inserted position, when the second portion of theshank is inserted in the bore of the head, the shank being insertableinto the head while the head is arranged in the receiving part, whereinin the first inserted position, the ring is positioned in the grooves ofthe head and the shank such that a surface of the ring engages acorresponding surface on the head or the shank, the engaging surfacesconfigured to form a positive-locking engagement that prevents slidingbetween said engaging surfaces and removal of the shank from the bore ofthe head when a force is applied to the shank in a direction opposite adirection of insertion, and wherein the receiving part has alongitudinal bore passing through it from a first end to a second end,and a U-shaped recess adjacent to the second end and intersecting thelongitudinal bore such that two free legs are formed constituting achannel for receiving the rod, the method comprising: attaching theshank to the bone or vertebra; and connecting the head to the shank byinserting the second portion of the shank in the bore of the head, theinserting of the second portion causing the inwardly tapered edgeportion to expand at least one of the ring and the head, such thatfurther insertion of the second portion of the shank in the bore of thehead causes the ring to engage the ring-shaped groove of the shank inthe first inserted position to connect the head to the shank.
 36. A boneanchoring system comprising: a plurality of shanks, any one of theshanks to be anchored in a bone or vertebra, at least one head, at leastone ring, and a receiving part receiving the head for connecting any oneof the shanks to a rod, the shanks, the head, and the ring beingseparate parts, the head having a bore with a cylindrical inner surfacedefining a smallest diameter of the bore, the cylindrical inner surfaceextending uniformly from a first end of the head to a second end of thehead and interrupted by a ring-shaped groove provided on the innersurface, and at least one slit extending from the inner surface to anouter surface of the head; and each shank having a first portion foranchoring in the bone or vertebra and a second portion with acylindrical outer surface, the cylindrical outer surface having aring-shaped groove; wherein the ring is configured to be arranged in thering-shaped groove of the head; wherein a free end of the second portionof one of the shanks comprises an inwardly tapered edge portion; whereinwhen the ring is arranged in the groove of the head, the ring isconfigured to engage the free end of any one of the shanks to expand thering by the inwardly tapered edge portion, and then to engage with thering-shaped groove of the shank in a first inserted position, when thesecond portion of the shank is inserted in the bore of the head, theshank being insertable into the head while the head is arranged in thereceiving part; wherein in the first inserted position, the ring ispositioned in the grooves of the head and the shank such that a surfaceof the ring engages a corresponding surface on the head or the shank,the engaging surfaces configured to form a positive-locking engagementthat prevents sliding between said engaging surfaces and removal of theshank from the bore of the head when a force is applied to the shank ina direction opposite a direction of insertion; and wherein the receivingpart has a longitudinal bore passing through it from a first end to asecond end, and a U-shaped recess adjacent to the second end andintersecting the longitudinal bore such that two free legs are formedconstituting a channel for receiving the rod.
 37. The bone anchoringsystem according to claim 36, wherein the shanks have different bonethreads.
 38. The bone anchoring system according to claim 36, wherein atleast one of the shanks comprises a coaxial bore and lateralthrough-holes extending from the bore to the outer surface of the shank.39. The bone anchoring system according to claim 36, further comprisinga plurality of heads, each head having a bore with a cylindrical innersurface defining a smallest diameter of the bore, the cylindrical innersurface extending uniformly from a first end of the head to a second endof the head and interrupted by a ring-shaped groove provided on theinner surface, and at least one slit extending from the inner surface tothe outer surface of the head providing elasticity to the head, whereineach head is connectable with any one of the shanks.
 40. The boneanchoring system according to claim 39, wherein the at least one slit ofat least one of the heads extends from a first end of the head to asecond end of the head.
 41. The bone anchoring system according to claim39, wherein several slits are provided in at least one of the headswhich alternately extend from a first end of the head to a predetermineddistance from a second end of the head and from the second end of thehead to a predetermined distance from the first end of the head,respectively.